D-Glucose,L-arabinose,p-mannose,D-xylose,and cellobiose are saccharification products of lignocellulose and important carbon sources for industrial fermentation.The fermentation efficiency with each of the five sugars and the mixture of the two most dominant sugars,D-glucose and D-xylose,was evaluated for acetonebutanol-ethanol(ABE) fermentation by Clostridium acetobutylicum ATCC 824.The utilization efficacy of the five reducing sugars was in the order of D-glucose,L-arabinose,D-mannose,D-xylose and cellobiose.D-Xylose,the second most abundant component in lignocellulosic hydrolysate,was used in the fermentation either as sole carbon source or mixed with glucose.The results indicated that maintaining pH at 4.8,the optimal pH value for solventogenesis,could increase D-xylose consumption when it was the sole carbon source.Different media containing D-glucose and D-xylose at different ratios(1:2,1:5,1.5:1,2:1) were then attempted for the ABE fermentation.When pH was at 4.8 and xylose concentration was five times that of glucose,a 256.9%increase in xylose utilization and 263.7%increase in solvent production were obtained compared to those without pH control.These results demonstrate a possible approach combining optimized pH control and D-glucose and D-xylose ratio to increase the fermentation efficiency of lignocellulosic hydrolysate.
A new hemA gene encoding 5-aminolevulinate (ALA) synthase was cloned from Agrobacterium ra- diobacter zju-0121. The ALA synthase catalyzes the pyridoxal phosphate-dependent condensation of succinyl coen- zyme A (succinyl-CoA) and glycine to produce ALA. Four plasmids carrying the A, radiobacter hemA gene were transformed into different E. coli strains. The effects of both genetic and physiological factors on the expression of ALA synthase and ALA production were studied. The results indicated that the final intracellular activity of ALA synthase and the production of ALA in different expression systems varied largely. Among them, the recombinant E. coli BL21 (DE3) harboring the expression plasmid pET28-A. R-hemA was the most suitable one. The effects of isopropyl-β-D-thiogalactopyranoside (IPTG) addition time, IPTG concentration, culture temperature and the initial concentration of precursors and glucose on the ALA production were also evaluated. The expressed ALA synthase accounted for about 23.7% of the intracellular soluble protein. The highest specific activity of ALA syn- thase was 13.8nmol·min-1·mg-1 of intracellular soluble protein. In the batch culture of the recombinant E. coli, the extracellular ALA concentration reached 0.9 g·L-1.
